Transitions among operating modes in an engine with a hybrid valvetrain

ABSTRACT

A method for controlling intake valves in an internal combustion with intake valves operated by more than one type of actuation device includes steps to accomplish smooth transitions among intake valve operating modes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of Application No. 09/650,434, filed Aug. 29, 2000.

FIELD OF THE INVENTION

The present invention relates to a control method for engine valveactuation.

BACKGROUND

In U.S. Pat. No. 6,009,841, an engine with a hybrid valvetrain isdisclosed in which one intake valve is actuated by a source other than acamshaft. This first intake valve is randomly operable meaning that thevalve opening and closing events are independent of engine crankshaftposition, thus, a fully variable valve. A second intake valve isactuated by a camshaft and includes a deactivator. Operation of thesecond valve may be discontinued or restored within one engine cycle,termed selectable intake valve herein. The exhaust valve(s) of thehybrid valvetrain is conventionally camshaft actuated. As disclosed inU.S. Pat. No. 6,009,841, the advantage of such a system over fullycamless engine operation is improved fuel economy.

In U.S. Pat. No. 6,009,841, the method is described in which air isadmitted using a randomly operable intake valve when the engine isoperating in a lowest range in torque, using a selectable intake valvewhen the engine is operating in a medium range in torque, and using boththe randomly operable intake valve and the selectable intake valve whenthe engine is operating in a highest range in torque.

In U.S. Pat. No. 5,647,312, a method is described in which air isadmitted using a randomly operable intake valve when the engine isoperating in a lower range in torque and speed and air is admitted usinga selectable intake valve when the engine is operating at higher speedor higher torque.

Both U.S. Pat. No. 6,009,841 and U.S. Pat. No. 5,647,312 teach thatmaking transitions among operating modes involves activating anddeactivating valves. The inventors herein have recognized that by simplyturning valves on and off leads to large excursions in engine torquewhich would be noticeable and annoying to the operator. The inventorsherein have also recognized that the potential for hybrid valvetrainfuel economy benefits, as disclosed in U.S. Pat. No. 6,009,841, dependson suitable methods to accomplish transitions between operating modes;otherwise, the fuel efficiency potential is unrealized.

SUMMARY OF THE INVENTION

A method for controlling an internal combustion engine, the enginecontaining at least one cylinder, a randomly operable intake valve, aselectable intake valve with a predetermined closing time, a throttlevalve in the engine's intake system, and an engine control unit, isprovided. The method includes the steps of providing a randomly operableintake valve closing time which is after the predetermined valve closingand initiating operation of the selectable intake valve. In a furtherstep, the throttle valve is closed and the randomly operable intakevalve closing time is advanced so that substantially constant enginetorque is provided.

A method for controlling an internal combustion engine, the enginecontaining at least one cylinder, a randomly operable intake valve, aselectable intake valve with a predetermined valve closing time, athrottle valve in the engine's intake system, and an engine controlunit, has a first step of activating the randomly operable intake valvewith a closing time after the predetermined valve closing time and asecond step of opening the throttle valve and retarding the randomlyoperable intake valve closing time.

A method for controlling an internal combustion engine, the enginecontaining at least one cylinder, a randomly operable intake valve, aselectable intake valve with a predetermined valve closing time, athrottle valve in the engine's intake system, and an engine controlunit, has a first step of advancing a randomly operable intake valveclosing time and closing the throttle valve and a second step of ceasingoperation of the randomly operable intake valve when additionaladvancement of the randomly operable intake valve closing time has noappreciable effect on inducted air remaining in the cylinder afterclosing of the randomly operable intake valve.

A method for controlling an internal combustion engine, the enginecontaining at least one cylinder, a randomly operable intake valve, athrottle valve in the engine's intake system, and an engine controlunit, has the steps of closing the throttle valve and advancing arandomly operable intake valve closing time, in the event that therandomly operable intake valve closing time is retarded from apredetermined valve closing time and closing the throttle valve andretarding a randomly operable intake valve closing time, in the eventthat the randomly operable intake valve closing time is advanced fromthe predetermined valve closing time. The predetermined valve closingtime is that which maximizes air inducted which is retained in thecylinder after closing of the randomly operable intake valve.

A method for controlling an internal combustion engine, the enginecontaining at least one cylinder, a randomly operable intake valve, athrottle valve in the engine's intake system, and an engine controlunit, has the steps of opening the throttle valve and advancing arandomly operable intake valve closing time, in the event that therandomly operable intake valve closing time is advanced from apredetermined valve closing time and opening the throttle valve andretarding a randomly operable intake valve closing time, in the eventthat the randomly operable intake valve closing time is retarded fromthe predetermined valve closing time.

The advantages of the above methods are numerous. The transitions amongoperating modes provide a smooth torque trajectory. Thus, thetransitions are not detectable by the operator. The time over which atransition occurs is brief, two to twenty engine cycles. The methodsdisclosed herein permit realization of the fuel efficiency potential ofan engine with a hybrid electric valvetrain.

Other advantages, as well as objects and features of the presentinvention, will become apparent to the reader of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a hybrid valvetrain engine showingcross-sections of the cylinder head and the fuel vapor recovery andpurge system to which aspects of the present invention apply;

FIG. 1A is a cross-section schematic of the cylinder head with thecross-section taken through the selectable intake valve;

FIG. 1B is a cross-section schematic of the cylinder head with thecross-section taken through the randomly operable intake valve;

FIG. 2 is a typical engine operating map onto which the regionsemploying the different modes of hybrid valvetrain operation areillustrated;

FIG. 3a is a graph of randomly operable intake valve lift profiles foradvanced and retarded closing times according to an aspect of thepresent invention;

FIG. 3b is a graph of intake valve closing time -showing the quantity offresh charge trapped in the cylinder;

FIG. 3c is a graph of intake valve closing time showing the trappedfresh charge reduction factor according to an aspect of the presentinvention;

FIG. 3d is a graph of intake valve closing time showing the temperatureof the trapped fresh charge;

FIG. 4a is a graph of valve lift profiles for both randomly operable andselectable valves;

FIG. 4b is a graph showing the effect on trapped fresh charge of varyingrandomly operable intake valve closing when the selectable intake valveis operated concurrently;

FIG. 5a illustrates a time history of throttle position for a transitionaccording to one aspect of the present invention;

FIG. 5b illustrates a time history of randomly operable intake valveclosing for a transition according to an aspect of the presentinvention;

FIG 5 c illustrates a time history of manifold absolute pressure for atransition according to an aspect of the present invention;

FIG 5 d illustrates a time history of mechanical valve status for atransition according to an aspect of the present invention;

FIG. 6 is a flowchart of steps involved in making a transition from aregion of lower engine speed and lower engine torque to other operatingconditions according to an aspect of the present invention;

FIG. 7 is a flowchart of steps involved in making a transition from aregion of higher engine speed and lower engine torque to other operatingconditions according to an aspect of the present invention;

FIG. 8a is a flowchart of steps involved in making a transition from aregion of higher engine torque to other operating conditions accordingto an aspect of the present invention;

FIG. 8b is a flowchart of steps involved in making a transition from aregion of higher engine torque to other operating conditions accordingto an aspect of the present invention;

FIG. 9 is a flowchart of steps involved in making a transition betweenmedium load in engine torque and lowest load in engine torque accordingto an aspect of the present invention; and

FIG. 10 is a flowchart of a method by which inputs of demanded enginetorque and rpm. can be used to compute intake valve closing and throttlevalve position as functions of time to command to actuators according toan aspect of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, engine 10 contains at least one cylinder 4. Thecylinder head of engine 10 contains selectable intake valve 18, randomlyoperable intake valve 16, and exhaust valves 20. Throttle valve 14 isdisposed in the intake 12 to engine 10. Combusted gases are rejectedthrough exhaust line 24. Engine control unit 26 is used to: activate anddeactivate the selectable intake valve 18, actuate the randomly operableintake valve 16, and control the position of the electronicallycontrolled throttle valve 14. Various engine sensors 28, such as anexhaust gas oxygen sensor, a mass air flow sensor, and an engine speedsensor, provide signals to the engine control unit 26.

Referring now to the cross-section shown in FIG. 1A, the selectableintake valve 18 and the exhaust valve 20 are camshaft actuated bycamshafts 2 and 3, respectively. As such, the timing of the events isbased on engine rotational position. In the cross-section shown in FIG.1B, the randomly operable intake valve 16 is driven by anelectromechanical actuator. An electrohydraulic actuator may also beused. The valve events of the randomly operable intake valve 16 arefully flexible and controlled by the engine control unit 26. The exhaustvalve 20 is actuated by camshaft 3. Also shown in FIGS. 1A and 1B are apiston 5 which reciprocates within the cylinder 4, an intake port 6, andan exhaust port 8. Intake ports 6 and exhaust ports 8 are coupled torespective intake and exhaust manifolds (not shown) to form respectiveintake line 12 and exhaust line 24.

A conventional fuel vapor recovery and purge system for an automotivetype engine also is shown in FIG. 1. Engine 10 communicates with anintake 12 and an exhaust 24. Fuel is metered into the intake by a fuelinjector 42. The throttle valve 14 is situated in the intake line 12.The fuel tank. 48 contains an in-tank fuel pump 50 which supplies fuelthrough a fuel supply line 44 to the fuel injector 42. The fuel tank 48is replenished through the fuel filler tube 46; fuel cap 68 is removedto allow filling. The liquid components of the fuel fall through tube62. Gaseous components may proceed through vapor recovery line 66.During fuel tank 48 filling, the volume not containing liquid fuel isoccupied by gaseous components which are pushed into the vapor recoverylines 64 and 66 by the entering liquid fuel. The vapor recovery lines 64and 66 lead to the carbon canister 52 which contains activated carbon toabsorb fuel vapors. The carbon canister 52 is purged regularly. Purgingis accomplished by opening valves 54 and 58 which allows fresh purge airto flow through the fresh purge air intake line 56. The gases exitingcarbon canister 52 contains both fresh air and fuel vapors which proceedthrough valve 58 and line 60. Line 60 is introduced into the intake 12downstream of the throttle valve 14. Flow from the carbon canistercircuit, through elements 56, 54, 52, 58, and 60, into the intake 12 andultimately to the engine 10 for combustion occurs under the conditionsof a vacuum in the intake 12 downstream of the throttle valve 14.

In FIG. 2, an operating map of a typical spark-ignited engine isdisplayed. The upper curve 40 represents the maximum torque that theengine delivers as a function of speed. Operating regions are shown atwhich the randomly operable valve operates along, the selectable valveoperates alone, and both valves operate. At higher torque across allspeeds, region 30, both intake valves are employed. Using both intakevalves admits the maximum air possible thereby allowing the engine todevelop its maximum torque. If intake air is admitted through one valvesolely, the velocity through that intake port and valve is roughly twiceas high as admitting the fresh charge through two valves, if the valvesare of comparable size. This leads to higher turbulence in the cylinderat the time of combustion. Although high turbulence is a desirablecondition at marginal combustion conditions, it leads to excessivelyrapid combustion or combustion harshness at robust operating conditionsexemplified by region 30. In region 36 of FIG. 2, the selectable intakevalve 18 only is employed. Region 36 does not require maximum airflow asmaximum torque is not demanded. Region 30 is to be selected when region36 cannot admit sufficient airflow or when the combustion is too harshusing the selectable intake valve 18 only.

Criteria are provided for determining when a transition is desired. Thedecision when to make a transition from region 36 to region 32 or region34 is based on whether the selectable intake valve 18 or the randomlyoperable intake valve 16 provides more efficient operation. Efficiencyis based on energy consumed in rotating the intake camshaft, energyconsumed in actuating the randomly operable intake valve 16, and pumpinglosses, that is, the energy consumed in replenishing spent combustiongases with a fresh charge.

Region 32 is a region in which the intake valve closing of the randomlyoperable intake valve 16 can be adjusted to match the demand for enginetorque. Lift profiles for the randomly operable intake valve 16 areshown in FIG. 3a. The intake valve closing time can be advanced orretarded from the timing which provides the maximum trapped freshcharge, as shown in FIG. 3b. By tailoring the intake valve closing time,the desired fresh charge is trapped. As shown in FIG. 3b, both retardingand advancing intake valve closing lessen the quantity of trapped freshcharge. In the case of retarded closing time, some of the inducted freshair charge is pushed out of the combustion chamber prior to intake valveclosing. The amount of fresh air retained in the cylinder is plotted inFIG. 3b.

At retarded intake valve closing times the fresh charge temperature isincreased; this is in contrast to advanced intake valve closing timeswhich have little effect on fresh charge temperature, as shown in FIG.3c. There may be reasons to prefer advancement or retardation of closingtime which will become apparent in the development of the method.Nevertheless, controlling engine torque by adjusting intake valveclosing time is preferred over throttling because it lessens pumpinglosses and hence leads to higher overall efficiency. As demanded torqueis reduced, engine torque control by adjusting the closing time of therandomly operable intake valve 16 may lead to unstable combustion.Depending on the combustion stability desired for a particularapplication, a torque level may be determined, below which engine torquecontrol is accomplished by throttling. Thus, the distinction betweenregions 32 and 34 is that throttling is employed within region 34.

Combustion stability is related to the standard deviation of the powerproduced within a cylinder on a cyclic basis. A low standard deviation,that is, constant power produced from cycle to cycle, indicates stablecombustion and vice versa. Herein, degradation in combustion stabilityrefers to an increase in the standard deviation and an improvement incombustion stability refers to a decrease in the standard deviation.

Within region 34 of FIG. 2, it is desirable to mitigate the level ofthrottling necessary to control engine torque. Thus, the intake valveclosing is as far advanced or retarded, depending on control methodbeing employed, as possible while maintaining satisfactory combustionstability. It may be found that as engine torque is reduced (withinregion 34), i.e., the throttle valve is closed, the intake valve closingtime must be altered to provide continued robust combustion.

As the demand for engine torque or engine speed changes during thecourse of operation, it will be found desirable to move between regionsin FIG. 2. The transition among the various regions of operation shouldbe imperceptible to the operator of the vehicle.

A transition from region 32 to 34 is provided when the combustionstability in region 32 becomes poorer than desired. A smooth transitionfrom region 32 to region 34 is accomplished by closing the throttlevalve to attain the desired torque level.

The transition from region 32 to region 30 is provided when the valveclosing time of the randomly operable intake valve 16 is at that whichprovides the maximum trapped fresh charge. It may be found that it isdesirable to cause a transition from region 32 to 30 based on a limitimposed by tolerable combustion harshness rather than running out ofauthority in intake valve closing time of the randomly operable valve.Further increase in engine torque is achieved by activating theselectable intake valve 18. Simultaneously, the randomly operable intakevalve time is retarded such that the air flow before and after thetransition cycle is substantially constant.

Engine torque control in region 30 is achieved by controlling the valveclosing time of the randomly operable intake valve 16. As illustrated inFIG. 2b, torque can be managed by late intake valve closing of therandomly operable intake valve 16, whereas, early intake valve closinghas minimal effect on the trapped charge.

A transition from region 36 to region 30 is desired when the selectableintake valve 18 alone does not supply sufficient fresh air charge. Whena transition is demanded, the randomly operable intake valve 16 isactivated with a retarded closing time in which trapped fresh charge isunaffected. The closing time of the randomly operable intake valve 16 isadvanced to increase trapped fresh charge mass as needed. The reversetransition (from region 30 to region 36) is accomplished when thetrapped fresh charge could be provided by the selectable intake valve 18alone. It may be found that instead of basing the transition on capacityconstraint the combustion harshness is the determining factor. That is,there may be operating conditions that the selectable intake valve 18alone can provide sufficient fresh charge, but the resulting combustionharshness is beyond a desired level. In this case, a transition fromregion 36 to region 30 is accomplished based on combustion harshness.

Region 32 or 34 is preferred over region 36 when the following equationholds true:

PWroiv+Wroiv+FW′siv+CLroiv>PWsiv+FWsiv+CLsiv

in which PWroiv is the pumping work of the engine with the randomlyoperable intake valve 16, Wroiv is the work extracted from the engine todrive the randomly operable intake valve 16, PWsiv is the pumping workof the engine using only the selectable intake valve 18, FWsiv is thefriction work lost in driving the selectable intake valve 18, FW′siv isthe friction work lost in driving the camshaft of the actuated intakevalve when the selectable intake valve 18 is deactivated. FW′siv isconsiderably less than FWsiv, but not negligible, due to rotationalfriction in the camshaft even when the selectable intake valve 18 isdeactivated. CLroiv and CLsiv are cycle losses associated with operatingthe randomly operable and selectable intake valves, respectively. Cyclelosses are difference between the ideal cycle work that could obtainedfor an Otto cycle and the actual amount generated. The actual workgenerated is less than ideal cycle work due to heat transfer, combustiontime losses (i.e., finite combustion duration), combustion phasing, andothers. The choice between using the selectable intake valve 18, region36, and using the randomly perable intake valve 16, regions 32 and 34,is based on minimizing the losses due to valve actuation and pumpingwork. When the above equation is violated, the control system selectsregion 36; i.e., the selectable intake valve 18 is actuated only.

The quantities described above may be computed or estimated in thefollowing ways. Pumping work (PW) is a function of manifold pressure,engine rpm, and engine displacement primarily and could be contained ina lookup table or in equation form in the engine's control unit. Theenergy loss associated with driving the intake camshaft is primarily afunction of engine rpm. This is a quantity which could be measured in arepresentative engine and the data applied to all engines of the sametype. This could be lookup table or an equation in the engine's controlunit. There would be two distinct tables or equations, one for the casewith the selectable intake valve 18 activated and for the case when itis deactivated. The power that is absorbed from the engine to actuatethe randomly operable intake valve 16 is a quantity that would bedetermined in the course of the development of the randomly variableintake valve. The design variables that would affect the powerrequirement is the size of the valve, the lift profile that is selected,and the drivers used to actuate the valve. For example, a faster valvelift expends more energy. The factors external to the valve design thatwould determine the power consumption is the efficiency of the enginealternator in generating electrical power, system losses in storing andretrieving energy, losses in transforming voltage, and pressure in thecylinder at the time of valve actuation. All of these quantities, exceptfor cylinder pressure, depend on the system design. Thus for a givendesign, the power consumed in actuating the randomly operable intakevalve 16 is a function of cylinder pressure primarily. Otherdependencies may be determined in- the course of development. Therandomly operable intake valve 16 alternatively may be actuatedelectro-hydraulically or by other means. In the electro-hydraulic case,power is consumed in driving a pump used to develop hydraulic fluidpressure, hydraulic losses in system lines (highly dependent onhydraulic fluid temperature), and electrical losses in actuatingcontrolling solenoid valves as well as the effect of the lift profile,valve size, and cylinder pressure effects mentioned above.

Accomplishing the transition between regions 32 or 34 to region 36 isshown in the timeline shown in FIG. 5. If at the time of the transition,the randomly operable intake: valve 16 is operating with an advancedclosing time, in the next engine cycle, a retarded closing time isselected which traps the same mass of fresh charge. In FIG. 3b, trappedfresh charge decreases on both sides of the peak. Thus, there is aretarded timing which can be selected in which trapped fresh charge and,thus, developed engine torque- matches that of the advanced timing. Inthe next engine cycle, the selectable intake valve 18 can be enabled.The trapped fresh charge is not appreciably affected by activating theselectable intake valve 18 when the randomly operable intake valve 16 isoperating at retarded valve closing time. Over the next engine cycles,the randomly operable intake valve closing time is advancedsimultaneously as the throttle valve is closed. These operations arecontrolled in concert such that the trapped fresh charge issubstantially constant herein meaning providing substantially constantengine torque or changing smoothly along the desired torque trajectory.A change in intake valve closing time of the randomly operable intakevalve 16 can be accomplished in a single engine cycle. In contrast, eventhough a change in throttle valve position can be accomplished rapidly,intake manifold filling considerations cause the intake manifoldpressure to react over several engine cycles. Thus, the transitionillustrated in FIG. 4 occurs in a matter of a few to a couple dozenengine cycles. As the timing of the randomly operable intake valve 16continues to be advanced, eventually it no longer has an affect on theamount of trapped fresh charge. At which point, it can be turned off.

The reverse transition (region 36 to region 32 or region 34 of FIG. 2)occurs analogously: the randomly operable intake valve 16 is activatedat an advanced timing in such a manner to not impact trapped freshcharge. The closing time of the randomly operable intake valve 16 isretarded concurrently with opening the throttle such that the trappedfresh charge is substantially constant. When the randomly operable valveclosing time is sufficiently retarded, the selectable intake valve 18 isno longer having any affect on the trapped fresh charge and may beturned off.

The distinction between making a region 32 to region 36 transition and aregion 34 to region 36 transition is that the initial throttle positionis fully open and the initial manifold pressure is atmospheric in theformer case and partially open, i.e., less than atmospheric, in thelatter case.

It is desirable to limit the number of transitions which the engine'scontrol unit must manage. Thus, as the demanded engine torque and speedapproaches a new region (within FIG. 2), the transition may be delayeduntil the demanded engine torque and speed traverse the border by apredetermined amount. The boundaries of FIG. 2 can be considered to bebands. As a boundary is approached, the transition is not made until thedemanded operating condition exceeds the farther edge of the boundary.That is, a transition from region 36 to region 30 would occur at thehigher engine torque edge of the boundary between the two regions.Conversely, a transition from region 30 to region 36 would occur at thelower engine torque edge of the boundary between the two regions.

Transitions among regions of FIG. 2 which involve closing or opening thethrottle valve, may require a minimum of one and as many as 20 enginerevolutions due to manifold filling lags. The throttle valve can beactuated on the order of 100 ms. However, causing air to fill themanifold takes multiple engine revolutions to overcome the inertia ofthe gases.

Herein, substantially constant engine torque means either a constanttorque or a trajectory in torque along the desired path; i.e., enginetorque deviation from the desired trajectory is small or unnoticeable tothe operator of the vehicle.

Retarding or advancing spark timing is a powerful tool which can be usedto smooth transitions. The advantage of spark advance is that it can bechanged in one engine cycle. Furthermore, spark timing has a wide rangeof authority in controlling engine torque. Spark timing, however,negatively impacts fuel economy, typically. Thus, it is a secondary toolto refine transitions.

A quantity which may be determined in development is rpm_(t) (identifiedin FIG. 2) which is the threshold rpm between regions 32 and 36. Thisquantity is discussed below in regard to the control strategy employedin selecting among regions for operation.

FIG. 3a shows valve lift profile for early and late closing time of therandomly operable intake valve 16. FIG. 3b indicates the resultingtrapped fresh charge as a function of the closing time of the randomlyoperable intake valve 16. A maximum trapped fresh charge occurs at aparticular valve closing time. At valve closing times advanced orretarded from that maximum reduce the quantity of trapped fresh charge.The amount of trapped fresh charge is the primary factor determining theamount of torque that the engine produces. When only the randomlyoperable intake valve 16 is operating, either a retarded timing or anadvanced timing may be selected to give a particular desired trappedfresh charge. The trapped fresh charge can be normalized by dividing thetrapped fresh charge at any given intake valve closing time. (IVC) bymaximum trapped fresh charge. The normalized quantity is termed trappedfresh charge reduction factor. As shown in 3 c, trapped fresh chargereduction factor ranges between 0 and 1.

When operating within region 34 of FIG. 2, the primary method by whichengine torque is controlled is by throttling and secondarily by intakevalve close timing. As shown in FIG. 3b, either an advanced or retardedvalve close timing could be used to provide a desired trapped freshcharge. In FIG. 3d, the resulting fresh charge temperature is shown. Atadvanced timings, the fresh charge temperature is substantiallyconstant; whereas, fresh charge temperature increases as a function ofretardation. This is due to the fact that at advanced timings, theintake valve is closed prematurely to limit the amount of trapped freshcharge. At retarded timings, the cylinder is filled with fresh chargeand as the piston moves up, the fresh charge is pushed out of thecylinder. In this case, the fresh charge comes in contact with the hotcylinder surfaces and hot intake valve multiple times and is heated morethan the advanced intake valve closing time case. Higher fresh chargetemperature may be found to be beneficial in improving combustionstability. Thus, within region 36, in which combustion stability is aconcern, retarded closing time of the randomly operable intake valve 16may be preferred due to improved combustion stability.

FIG. 4a shows the valve lift profiles for operation with both theselectable intake valve 18 and the randomly operable intake valve 16.FIG. 4b indicates the resulting trapped fresh charge as a function ofthe closing time of the randomly operable intake valve 16. A maximumtrapped fresh charge occurs at a particular valve closing time of therandomly operable intake valve 16. At times advanced of the maximum, avery slight reduction in trapped fresh charge may occur. That is, therandomly operable intake valve 16 does not have substantive authorityover trapped fresh charge by advancing the closing time. In FIG.4b, therandomly operable intake valve 16 closing time does have a range ofauthority over trapped fresh charge when the closing time is retardedbeyond the closing time of the selectable valve, i.e., retarded timing.

In FIG. 2, the regions among which the engine control unit must selectoperation are illustrated. In addition to providing control within eachregion, smooth transitions between regions must be managed. A time lineof a transition between region 32 to region 34 is outlined in FIGS.5a-d. If the randomly operable intake valve 16 is operating at anadvanced timing 50, it must be switched to a retarded timing whichprovides identical trapped fresh charge at the start of the transition,as shown in FIG. 5b. The ability to find a retarded timing whichprovides the same trapped fresh charge as an advanced timing issupported by FIG. 3a, as discussed above. If the randomly operableintake valve 16 is operating at a retarded timing 52, no action isnecessary. The selectable valve can be opened in the same engine cycleor shortly thereafter, FIG. 5c. The throttle valve (designated TP forthrottle position in FIG. 5a) is closed. Simultaneously, the randomlyoperable intake valve closing time is retarded, FIG. 5d, such that thedesired trajectory in engine torque is achieved. As the randomlyoperable intake valve timing is retarded beyond a certain point, it isno longer impacting the quantity of trapped fresh charge. At this point,the randomly operable intake valve 16 can be turned off, FIG. 4b.

For the purposes of discussing the control strategy, region 30 of FIG. 1is called region three, region 36 of FIG. 1 is called region two, andthe combined region containing both regions of 32 and 34 of FIG. 2 iscalled region one. For purposes of discussing control strategy in FIG.9, region 32 and 34 of FIG. 2 are termed regions four and five,respectively.

FIG. 6 indicates the steps that would be taken to assess whether atransition from region one were called for and then to make thetransition. The system is operating in region one in block 100. Blocks102, 104, and 106 are assessment steps as to whether a transition iswarranted. The order in which assessment blocks 102, 104, and 106 occuris arbitrary. In block 102, the losses operating in region 1 compared tolosses operating in region two are evaluated. These losses are describedabove as consisting of all losses associated with operating theselectable intake valve 18 compared to the randomly actuated intakevalve. The object of block 102 is to select the more efficient operatingregion. If region two is more efficient, a transition from region one toregion two is accomplished beginning in block 108. The intake valvetiming of the randomly operable intake valve 16 is compared to thatwhich would give maximum trapped fresh charge in block 110. If thetiming is advanced from the maximum trapped fresh charge condition, theintake valve timing is changed to a retarded intake valve timing whichgives substantially similar trapped fresh charge in block 112. Theselectable intake valve 18 is activated in block 114. The purpose forthe change from an advanced timing to a retarded timing in block 112 isillustrated in FIGS. 3 and 4. With only the randomly operable intakevalve 16 activated as in FIG. 3, trapped fresh charge drops on bothsides of the maximum. But, with both valves open as in FIG. 4, trappedfresh charge remains substantially constant on the advanced side anddrops on the retarded side. To allow valve timing a range in authorityin controlling trapped fresh charge, the randomly operable intake valve16 should be operating at a retarded timing. In block 116, the throttlevalve is closed concurrently with advancing the valve closing time ofthe randomly operable intake valve 16. This is accomplished such thatengine torque is substantially constant. Block 118 is a check todetermine whether the closing time of the randomly operable intake valve16 is sufficiently advanced such that it no longer has authority overfresh trapped charge. If not, block 116 is repeated. When the test inblock 118 fails, the randomly actuated intake valve can be turned off inblock 120 because it no longer affects engine torque. Block 122indicates that the engine is operating within region two.

In block 102 of FIG. 6, if losses in region one are less than regiontwo, checks 104 and 106 are made. In block 104, a check is made todetermine if the randomly operable intake valve 16 admits sufficienttrapped fresh charge so that demanded engine torque can be satisfied. Ifnot, a transition from region one to region three is requested in block124. If the test of block 104 is passed, an additional test is made inblock 106 to determine if the combustion harshness is acceptable.Because the randomly operable intake valve 16 induces more turbulence inthe combustion gases than when intake gases are inducted through bothvalves, the resulting combustion can become too rapid or harsh. In block106, existence of harsh combustion causes a request for a transitionfrom region one to region three. The selectable intake valve 18 isturned on after the randomly operable intake valve close timing isretarded in block 126. The closing time for the randomly operable intakevalve 16 is selected so that substantially constant engine torque isachieved during the transition of block 130. Now the engine is operatingin region three in block 128.

FIG. 7 shows steps involved in checking for and making transitions fromoperating region two, block 200. The checks to determine if a transitionis warranted are blocks 202, 204, and 206, which can performed in anyorder. Block 202 checks whether region one or two is more efficient. Ifregion one is more efficient, a transition from region two to region oneis requested, block 208. The randomly operable intake valve 16 isactivated with a closing time advanced of maximum trapped fresh chargein block 210. By advancing the closing time, the randomly operableintake valve 16 has a minimal effect on trapped fresh charge. Theclosing time is retarded in block 212 until in block 214 furtherretardation would affect trapped fresh charge. When block 214 issatisfied, the randomly operable intake valve closing time is retardedconcurrently with opening the throttle valve, block 216. This isaccomplished such that engine torque remains substantially constantuntil the desired throttle opening is achieved. The selectable intakevalve 18 may be turned off as shown in block 218. Blocks 216 and block218 may be accomplished in arbitrary order. Now the engine is operatingin region one, block 220.

In block 202 of FIG. 7, if the losses of region two re less than thoseof region one, checks are made in blocks 204 and 206 to determinewhether sufficient engine torque can be developed using the selectableintake valve 18 alone and whether combustion harshness is acceptable,respectively. If either of these checks fails, a transition from regiontwo to region three is demanded, block 222. The randomly operable intakevalve 16 is turned on with an advanced closing time. The valve closingtime is retarded in block 226 with a check to see if further retardationwill affect trapped fresh charge, block 228. When the test of block 228passes, the intake valve timing is retarded further while the throttlevalve is opened in such a manner to maintain substantially constantengine torque in block 230. The throttle is opened fully or to a lesseropening depending, if desired to support other functions. The engine isnow operating within region three, block 232.

FIG. 8a shows steps involved in checking for and making transitions fromoperating region three, block 300. Blocks 302, 304, 306, 316 and 318 arechecks to determine if a transition is warranted. In block 302, thecurrent rpm is compared to rpm_(t), which is a predetermined value ofrpm which indicates the boundary between regions one and two (rpm_(t) isshown in FIG. 2.) The purpose of step 302 is to determine if atransition is made whether it is to region one or region two. If rpm isless than rpm_(t), then checks in block 304 and 306 determine whetherenough engine torque can be produced and whether combustion harshnesswould be acceptable with the randomly operable intake valve 16. Ifeither check 304 or 306 fail, control is returned to block 300. If bothchecks 304 and 306 pass, a transition to region one is made in block308. In block 312, the selectable intake valve 18 is closed, therandomly operable intake valve 16 is advanced to maintain engine torque.If rpm is greater than rpm_(t), checks 316 and 318 are made to determinewhether enough engine torque can be produced and whether combustionharshness would be acceptable with the selectable intake valve 18.Failure in either check 316 or 318 returns control to block 300,operating region three. If both check 316 and 318 pass, a transition toregion two is requested in block 320. The throttle valve is closed andthe closing time of the randomly operable intake valve 16 is advanced inblock 322. A check in 324 determines whether further advancement of theclosing time of the randomly operable intake valve 16 affects trappedcharge. If so, return to block 322. If not, deactivate the randomlyoperable intake valve 16 in block 326 and the system is now operatingwithin region two in block 328.

FIG. 8b indicates an alternative to FIG. 8a. Blocks 302, 304, 306, 316,and 318 of FIG. 8a are replaced with blocks 350, 352, and 354 of FIG.8b. In block 350, four questions with binary responses are asked.Question one is “can enough engine torque be produced with theselectable intake valve 18.” Question two is “can enough engine torquebe produced with the randomly operable intake valve 16.” Question threeis “whether combustion harshness would be acceptable with the selectableintake valve 18.” Question four is “whether combustion harshness wouldbe acceptable with the randomly operable intake valve 16.” Block 352shows the direction of control based on the responses of the fourquestions. Control proceeds along path A if the four answers are allpositive or yes. Path A leads to block 354 in which a check is made todetermine if the losses in region one are less than losses in regiontwo. If positive result from block 354, a transition from region threeto region one is demanded, block 308. If negative, a transition fromregion three to region two is demanded, block 320. Control proceedsalong path B if the answers to questions one and three are positive andeither one or both of the answers to questions two and four arenegative. Path B calls for a transition from region three to region two,block 320. Similarly, a positive answer to both questions two and four(with either one or both answers to questions one and three negative)leads to control along path C which leads to block 312, a transitionfrom region three to region one. Any other result than those discussedabove, leads to result D which is a return to block 300, operatingregion three. The remaining control steps of FIG. 8b are discussed abovein regard to FIG. 8a.

In FIG. 9, if operating within region four, block 400, a transitionwould be requested when combustion stability is unacceptable, block 402.To make the transition block 404, the throttle valve is closed while therandomly operable intake valve closing time is advanced maintainingsubstantially constant engine torque, block 406. The engine is operatingwithin region five, block 408.

If operating within region five, block 420 of FIG. 9, a check is made todetermine if combustion stability would be acceptable withoutthrottling, block 422. If so, a transition from region five to regionfour is requested, block 424. The throttle is opened while intake valveclosing time is retarded such that substantially constant engine torqueis developed during the transition in block 426. The engine is nowoperating within region four, block 428.

In FIG. 8, checks are made in blocks 316 and block 304 to determinewhether enough engine torque can be produced with the selectable intakevalve 18 and the randomly operable intake valve 16, respectively. Checksare made concerning combustion harshness in blocks 306 and 318. It maybe found in the course of development that combustion harshness, torqueproduction, or other measure is the sole criteria by which a transitionshould be demanded. The strategies discussed in relation to FIGS. 6-9may be simplified in accordance.

An algorithm to calculate throttle position and intake valve closing fortransitions involving a throttle change to be disposed in the electroniccontrol unit is outlined below. Specifically, transitions involving athrottle valve change are any transitions involving region 36 andtransitions between regions 32 and 34 of FIG. 2. Desired trapped freshcharge (des_trp_chg) depends on demanded or desired engine torque(des_tq) and engine speed (rpm), i.e.,

des _(—) trp _(—) chg=fnc(des _(—) tq, rpm).

In an engine with fixed valve events, des_trp_chg, and desired manifoldpressure (des_MAP) are related by

des _(—) MAP=a*des _(—) trp _(—) chg+b

where a and b are functions of rpm.

In an engine with flexible valve events, the effect of valve timing canbe included as

des _(—) MAP=c*des _(—) trp _(—) chg/trp _(—) chg _(—) rf+d  (1)

where c and d are functions of rpm and trp_chg_rf is a trapped freshcharge reduction factor, defined as.

trp _(—) chg _(—) rf=trp _(—) chg(IVC)/trp _(—) chg(IVC _(m))

where trp_chg(IVC) is the trapped fresh charge at the given IVC andtrp_chg(IVC_(m)) is the trapped fresh charge at IVC_(m), which is theIVC which gives the maximum trapped fresh charge. IVC is intake valveclosing time. It is apparent from FIG. 3c, that trp_chg_rf rangesbetween 0 and 1 and that

trp _(—) chg _(—) rf=fnc(IVC)

at a given MAP and rpm.

Or, in the general case, trp_chg_rf=fnc (IVC, MAP, rpm), the detailedform of the equation will be determined in development.

Solving for trp_chg_rf in equation 1 above,

trp _(—) chg _(—) rf=(c*des _(—) trp _(—) chg)/(des _(—) MAP−d)=fnc(IVC,MAP, rpm).

As mentioned above, the relationship between IVC and trp_chg_rf is notknown, a priori. However, given such a relationship, the equation can besolved for IVC. IVC depends on

IVC=fnc(, MAP, des _(—) trp _(—) chg, rpm).

The desired throttle position (TP) is related to MAP through sonic andsubsonic relationships. These relationships are known to one skilled inthe art and are the subject of U.S. Pat. No. 5,526,787, which isassigned to the assignee of the present invention and which isincorporated by reference herein. It is covered, also, within “InternalCombustion Engine Fundamentals” by J. B. Heywood (McGraw Hill, 1988),which is hereby incorporated by reference herein.

TP=fnc(MAP, rpm).

The relationships above hold for a single operating condition. However,transitions which involve opening or closing the throttle valve (betweenregion 36 and another other region of FIG. 2) occur over an interval andrequire concurrent ramping of IVC and throttle position. The inventionherein uses a ramp in MAP to define ramps in IVC and TP. MAP is rampedbased on the desired final MAP (des_MAP) and the current or initial MAP(MAP_(i)). A change in throttle position, with a typical electronicallycontrolled throttle valve, can occur much faster than the manifoldpressure can react due to the inertia of the gases. Depending on themagnitude of the change desired and engine speed, it may take from aboutone to twenty engine cycles for the manifold pressure to reach itsequilibrium level. The desire for smooth transitions among regions ofFIG. 2 suggests that the ramp in MAP be sufficiently slow such thatmanifold filling lag is minimal. A linear ramp in MAP may be preferredwith the end points defined by MAP_(i) and des_MAP and the slopedetermined by manifold filling considerations. The ramp in MAP isMAP(t). The ramps in both IVC and TP are based on this ramp in MAP withthe additional constraint of delivering des_trp_chg. Thus, IVC(t) andTP(t) can be computed based on manifold pressure ramp and desiredtrapped fresh charge:

IVC(t)=fnc(MAP(t), des _(—) trp _(—) chg, rpm)

and

TP(t)=fnc(MAP(t), des _(—) trp _(—) chg, rpm)

with rpm as a given.

In FIG. 10, the steps in computing IVC(t) and TP(t) are outlined. Inputsto block 500 are the desired or demanded torque, des_tq, and enginespeed, rpm. Within block 500, the desired trapped fresh charge(des_trp_chg) is computed. In block 502, the desired manifold pressure(des_MAP), that is, the final MAP at the completion of the transition,is computed with des_trp_chg, rpm, and the operational status of theintake valves at the end of the transition. The operational status ofthe intake valves for a transition from region 36 to region 30 of FIG. 2is for both the randomly operable intake valve 16 and the selectableintake valve 18 to be activated. Within block 504, the ramp in MAP,des_MAP(t), is computed with inputs of initial MAP, MAP_(i), anddes_MAP. As discussed above, a MAP trajectory may be linear, block 512,and takes into account intake manifold filling considerations. Thedesired MAP trajectory characteristics may be reduced to an algorithmand disposed within the engine's control unit. The output of block 504,des_MAP(t), along with des_trp_chg are inputs to both blocks 506 and 508in which IVC(t) and TP(t) are computed, respectively. Typical automotiveengine control systems incorporate a measure of delivered fresh charge,shown as block 510. Measured fresh charge is input to block 510 in whichtrapped fresh charge can be computed. The actual trapped fresh charge isinput to block 508 allowing error checking and updating of the throttleposition equations or lookup tables.

Referring again to FIG. 1, modern automotive vehicles are equipped withvapor recovery and purge systems to manage fuel vapors evolving from theliquid fuel in the fuel tank 48 due to temperature cycling and due tofuel vapors that are displaced in the process of refilling the fueltank. The system contains a carbon canister 52 which absorbs the fuelvapors. When a purge is called for by the engine's electronic controlunit 26, fresh air is drawn through the canister 52. The fresh air anddesorbed vapors are inducted into the engine entering the engine on thedownstream side of the throttle valve 14. Purge vapors are drawn throughthe engine by virtue of the vacuum or depressed pressure in the intake12.

As conventional automotive spark-ignition, internal combustion engines10 are throttled under most operating conditions, scheduling carboncanister purge is not usually an impediment. Additional measures must betaken to purge an internal combustion engine 10 with a hybridvalvetrain. Within region 32 (FIG. 2), in an internal combustion engine10 with a hybrid valvetrain the throttle 14 is open. Thus, there is novacuum in the intake 14 (FIG. 1). If the engine control unit calls for apurge, the throttle 14 may be closed to accommodate the need to purgethe carbon canister 52. To overcome the torque reduction caused byclosing the throttle 14, the closing time of the randomly actuatedintake valve 16 is changed to allow induction of sufficient trappedfresh charge to meet the demand for engine torque. When the enginecontrol unit determines that the carbon canister 52 has been purged, thethrottle valve 14 may be reopened while concurrently altering theclosing time of the randomly actuated intake valve 16 to meet demandedengine torque. A transition to purging while operating within region 32of FIG. 2 can be accomplished in the same manner as a transition fromregion 32 to region 34 as discussed above. The distinction is that thetransition is requested based on a need to purge the vapor recoverysystem rather than combustion stability. Analogously, a transition froma purge is accomplished the same as a transition from region 34 toregion 32 described above.

In high torque operating region 30 in FIG. 2, throttling is notemployed. It may be possible to schedule purging of the fuel vapor purgesystem such that sufficient purge time would be scheduled outside region30. In normal engine operation, region 30 is rarely accessed. However,if purging were desired, the approach described above for purging region32 would apply to region 30 with the distinction that region 30 cannottolerate as much throttling due to the need to develop high torque.

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize alternative designs and embodiments for practicing theinvention. Thus, the above-described preferred embodiment is intended tobe illustrative of the invention, which may be modified within the scopeof the following claims.

What is claimed is:
 1. A method for controlling an internal combustionengine, the engine having at least one cylinder, at least one randomlyoperable intake valve, a throttle valve disposed in an air intake systemof said engine, and an engine control unit, said method comprising thesteps of: closing the throttle valve and advancing a closing time ofsaid randomly operable intake valve, in the event that said closing timeof the randomly operable intake valve is retarded from a predeterminedvalve closing time; and closing the throttle valve and retarding aclosing time of the randomly operable intake valve, in the event thatsaid closing time of the randomly operable intake valve is advanced fromsaid predetermined valve closing time.
 2. The method according to claim1, wherein said predetermined valve closing is a closing time whichmaximizes air inducted which is retained in the cylinder after closingof the randomly operable intake valve.
 3. The method according to claim1, wherein said closing of the throttle valve and said closing time ofthe randomly operable intake valve are selected so that engine torque issubstantially constant.
 4. The method according to claim 1, wherein afirst operating mode precedes said closing of the throttle valve, saidfirst operating mode characterized by a first relationship betweenengine speed and engine torque.
 5. The method according to claim 4,wherein a second operating mode succeeds said closing of the throttlevalve, said second operating mode characterized by a second relationshipbetween engine speed and engine torque wherein for a given engine speedsaid first operating mode provides a higher engine torque than saidsecond operating mode.
 6. A method for controlling an internalcombustion engine, the engine having at least one cylinder, at least onerandomly operable intake valve, a throttle valve disposed in an intakevalve system of said engine, and an engine control unit, said methodcomprising the steps of: opening the throttle valve and advancing aclosing time of the randomly operable intake valve, in the event thatsaid closing time of the randomly operable intake valve is advanced froma predetermined valve closing time; and opening the throttle valve andretarding said closing time of the randomly operable intake valve, inthe event that said closing time of the randomly operable intake isretarded from said predetermined valve closing time.
 7. The methodaccording to claim 6, wherein said predetermined valve closing time is aclosing time which traps maximum fresh charge.
 8. The method accordingto claim 6, wherein said opening of the throttle valve and said closingtime of the randomly operable intake valve are selected so that enginetorque is substantially constant.
 9. The method according to claim 6,wherein a first operating mode precedes said opening of the throttlevalve, said first operating mode characterized by a first relationshipbetween engine speed and engine torque.
 10. The method according toclaim 9, wherein a second operating mode succeeds said opening of thethrottle valve, characterized by a second relationship between enginespeed and engine torque, wherein at a given engine speed said secondoperating mode provides higher engine torque than said first operatingmode.
 11. A system for controlling an internal combustion engine, theengine having at least one cylinder and a throttle valve disposed in anair intake, comprising: a randomly operable intake valve coupled to thecylinder; and an electronic control unit operably connected to theengine, the throttle valve, and said randomly operable intake valve,said electronic control unit commands the throttle valve to close andcommands said randomly operable intake valve to close at an advancedtiming when said closing time of said randomly operable intake valve isretarded from a predetermined valve closing time.
 12. The system ofclaim 11, wherein said electronic control unit commands said randomlyoperable intake valve to close at a retarded timing when said closingtime of said randomly operable intake valve is advanced from apredetermined valve closing time.
 13. The system of claim 12 whereinsaid predetermined valve closing time is a closing time trapping maximumfresh air in the cylinder.
 14. The system of claim 12 wherein a firstoperating mode precedes said closing of the throttle valve, a secondoperating mode follows said closing of the throttle valve, and saidfirst operating mode provides a higher engine torque than said secondoperating mode.
 15. The system of claim 14 wherein said closing time ofsaid randomly operable intake valve provides a smooth torque trajectorybetween said first operating mode and said second operating mode.
 16. Asystem for controlling an internal combustion engine, the engine havingat least one cylinder and a throttle valve disposed in an air intake,comprising: a randomly operable intake valve coupled to the cylinder;and an electronic control unit operably connected to the engine, thethrottle valve, and said randomly operable intake valve, said electroniccontrol unit commands the throttle valve to open and commands saidrandomly operable intake valve to close at a retarded timing when saidclosing time of said randomly operable intake valve is retarded from apredetermined valve closing time.
 17. The system of claim 16, whereinsaid electronic control unit commands said randomly operable intakevalve to close at an advanced timing when said closing time of saidrandomly operable intake valve is advanced from a predetermined valveclosing time.
 18. The system of claim 17 wherein said predeterminedvalve closing time is a closing time trapping maximum fresh air in thecylinder.
 19. The system of claim 17 wherein a first operating modeprecedes said closing of the throttle valve, a second operating modefollows said closing of the throttle valve, and said first operatingmode provides a lower engine torque than said second operating mode. 20.The system of claim 19 wherein said closing time of said randomlyoperable intake valve provides a smooth torque trajectory between saidfirst operating mode and said second operating mode.